This model uses the diskbb parameterisation of disc temperature
with radius i.e. \(T\propto r^{-3/4}\)  [1]}, so does not
include a stress-free inner boundary condition or colour temperature
correction. If the disc is truncated then the lack of stress-free
inner boundary condition is probably more appropriate, but the colour
temperature correction, \(f_{col}\) , is still an issue. The derived
value of the inner radius is then underestimated by a factor
\(f^2_{col}\sim 2.9 \)  for \(f_{col}\sim 1.7\)  [2]}.
However, the main aim of our spectral fitting is to track changes in the value of the inner disc radius, as its absolute value
depends on the poorly known system parameters. The key aspect of the
truncated disc model which we are testing is the prediction that the
truncation radius increases as the spectrum hardens during the
decline. Thus we are focussing on the relative values of the inner
disc radius during the decline.
